1. Field of the Invention
This invention relates to novel hop acid compounds that provide improved flavor, foam, and antimicrobial contributions in malt beverages such as beer and active ingredients for health supplements. In particular, the invention relates to methods for preparing a hop acid mixture having an enantiomeric excess of a (+)-tetrahydro-α-acid, methods for preparing (+)-tetrahydro-α-acids that can be isomerized to (+)-trans-tetrahydro-iso-α-acids and (−)-cis-tetrahydro-iso-α-acids, and reduced to (+)-trans-hexahydroiso-α-acids and (−)-cis-hexahydroiso-α-acids and malt beverage bittering agents including the (+)-trans-tetrahydro-iso-α-acids, (−)-cis-tetrahydro-iso-α-acids, (+)-trans-hexahydroiso-α-acids, (−)-cis-hexahydroiso-α-acids, or mixtures thereof.
2. Description of the Related Art
Chiral recognition of substances, i.e. the ability to distinguish a molecular structure from its mirror image, is one of the most important and widespread principles of biological activity. The first molecular event in odor perception is the interaction of an odorant with a receptor. As olfactory receptors have been identified as proteins, i.e. chiral molecules, this interaction should also be enantioselective, meaning that odor receptors should react differently with the two enantiomeric forms of a chiral odorant, leading to differences in odor strength and/or quality. Discrepant enantiomer effects are well-established, with numerous examples in taste perception. For example, limonene is present in both orange and lemon peels and responsible for their different odor characteristics because orange contains the right-handed (+) molecule while lemon contains the left-handed (−) molecule; (S)-(+)-carvone is a molecule with caraway-like odor while its mirror image molecule (R)-(−)-carvone has a spearmint odor. Linalool is one of main key hop flavor components in beer, which optical isomers have great impact on the character of hoppy flavor. (−)-Linalool is perceived with woody, lavender-like aroma, while its mirror image molecule, (+)-linalool, has sweet and citrus-like aroma.
Tetrahydroiso-α-acids (including three major analogs of tetrahydroisocohumulone, tetrahydrohumulone, and tetrahydroadhumulone) have shown more benefits in brewing than their analogous of iso-α-acids, ρ-iso-α-acids, and hexahydroiso-α-acids. Tetrahydroiso-α-acids impart the most bitter intensity, provide more light stability and flavor stability, enhance more foam, and exhibit stronger antimicrobial activity than the other hop bittering compounds in beer. Tetrahydroiso-α-acids are prepared from either α-acids (including three major analogs of cohumulone, n-humulone, and adhumulone) or β-acids (including three major analogs of colupulone, n-lupulone, and adlupulone) (See, P. Ting & H. Goldstein, J. Am. Soc. Brew. Chem. 54(2):103-109, 1996). From the α-acids (humulones), sequential hydrogenation and isomerization reactions or reversed isomerization and hydrogenation reactions of α-acids are involved as shown in FIG. 1, wherein R═CH2CH(CH3)2 for n-humulone, R═CH(CH3)2 for cohumulone, and R═CH(CH3)CH2CH3 for adhumulone. From the β-acids (lupulones), multiple reactions are involved including a sequential hydrogenolysis/hydrogenation reaction of β-acids, oxidation reaction of the hydrogenated desoxy-α-acids and then an isomerization reaction of tetrahydro-α-acids as shown in FIG. 1, wherein R═CH2CH(CH3)2 for n-lupulone, R═CH(CH3)2 for colupulone, and R═CH(CH3)CH2CH3 for adlupulone.
Both methods produce identical molecules of tetrahydroiso-α-acids, but only different from their stereoisomers. Tetrahydroiso-α-acids prepared from α-acids are optically active compounds, or enantiomers, due to the natural structure of α-acids (asymmetry molecules) (see, D. De Keukeleire and M. Verzele, J. Inst. Brewing, 76:265, 1970). However, tetrahydroiso-α-acids prepared from β-acids (no asymmetry molecules) are a racemic mixture (containing pairs of mirror image molecules or equal opposite enantiomers) with no optical activity (see, Patrick L. Ting and Henry Goldstein, J. Am. Soc. Brew. Chem. 54(2):103-109, 1996).
The molecular perception of stereochemistry of tetrahydroiso-α-acids and hexahydroiso-α-acids prepared from either α-acids or β-acids is very important because of their potential flavor, foam, antimicrobial contributions in beer as well as important ingredients for nutraceuticals and functional food (see U.S. Pat. No. 7,270,835). However, the stereochemistry and physiological properties (chiral recognition) have not been investigated and reported for tetrahydroiso-α-acids prepared from β-acids.
Therefore, there still exists a need for tetrahydroiso-α-acid compounds that improve flavor, foam, and antimicrobial contributions in malt beverages such as beer.